They can adapt slowly, although Putin's need to tolerate massive corruption in Russian military procurement and in the army in exchange for political support limits his ability to bolster his military.
If the Ukrainians are forced to settle along current lines, Russia will continue building its forces and will try again soon, which is why it must be defeated and at least forced back to its Feb 22 positions.
Yes, that’s definitely an advantage, except that Russia seems to have wasted a lot of equipment in the early stages of the war and doesn’t seem to be very good at logistics either. The bigger problem I see for Russia is just a general lack of strategic acumen. The successes they manage to achieve seem solely due to brute force, at great cost, which is tremendously wasteful in a long conflict compared to making gains by pitting your strengths against the opponent’s weaknesses, which is what Ukraine started doing and what brought them most of their recent gains.